1. Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus having a function of measuring colors of a measurement image.
2. Description of the Related Art
Image quality of an image forming apparatus includes granularity, in-plane uniformity, text quality, and color reproducibility (including color stability). In recent years in which a multi-color image forming apparatus has become widely used, the color reproducibility in some cases is the most significant image quality.
Humans have a memory of expected colors (in particular, of a human skin, blue sky, and metal) based on experience, and if a color exceeds an allowable range of such expected color, a viewer develops a feeling of strangeness. Such color is referred to as a memory color, and reproducibility of the memory color has become a concern when outputting a photograph.
Further, increasing demand for color reproducibility (including image stability) with respect to the image forming apparatus is not limited to the photographic image. In the case of a document image, there is an increasing demand from office users who feel discomfort in a color difference between the output from the image forming apparatus and that on a monitor, and from graphic artists who pursue color reproducibility of a computer graphics (CG) image.
To satisfy such user demands for color reproducibility, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus which reads, using a measurement unit (i.e., a color sensor) disposed in a conveyance path of a sheet, the measurement image (i.e., a patch image) formed on the sheet. Such an image forming apparatus feeds back into process conditions such as an exposure amount and a developing bias, a result obtained by the color sensor reading the patch image. Constant density, gradation, and tint can thus be reproduced.
However, color detection accuracy of the color sensor discussed in Japanese Patent Application Laid-Open No. 2004-086013 becomes degraded by an output fluctuation of a light source due to a change in environmental temperature. To solve such a problem, a white reference plate may be arranged opposite to the color sensor, so that the color sensor measures the white reference plate and corrects a detection value of the color sensor.
However, if there is soiling of a window portion of the color sensor or the white reference plate, scattered light from the soiled portion causes an error in the measurement result. In such a case, it becomes necessary to notify service personnel to remove the soiling. However, if the soiled portion cannot be identified, the service personnel may clean or replace an unsoiled component. As a result, working hours of the service personnel may become long, and running cost may increase due to replacement of the component.